A 44 year old woman presented to an outside hospital with a chief complaint of abdominal tenderness and increased abdominal girth over the past few days. Her past medical history was significant for type II diabetes mellitus and associated complications including retinopathy and chronic kidney disease. As a result of her renal failure, she was currently undergoing peritoneal dialysis. Due to concern for infectious peritonitis, a paracentesis was performed and the resulting fluid obtained was sent to the Microbiology laboratory for Gram stain and bacterial culture. Because of difficulty in identifying the isolate, it was sent to our mycology section for further work up.
Figure 1. Many sporangia of various sizes containing endospores arranged in a “soccerball” like pattern or morula configuration (Lactophenol cotton blue prep, 400x).
The isolate was received on potato dextrose agar (PDA) and appeared as discrete cream colored colonies, resembling a yeast. Upon transfer to cornmeal agar and incubation at 30°C, the isolate grew rapidly over the course of two days. The organism did not grow on media containing cycloheximide. A lactophenol cotton blue prep was performed (Figure 1) and showed many sporangia, ranging in size from 4-15 µm. In the larger forms, individual endospores were able to visualized and they were arranged in a “soccerball” like pattern. No budding or hyphae were present. Given these characteristics, the organism was identified as Prototheca wickerhammii.
Prototheca wickerhammii is classified as an achlorophyllous algae and is known to cause human infections involving the skin and subcutaneous tissues, bursa of the elbow joint (olecranon bursitis) and rarely, systemic infections. P. wickerhammii is ubiquitous in nature and infection is usually the result from traumatic inoculation. Both immunocompetent and immunocompromised hosts can be affected, although more severe or systemic disease occurs in those who have defects in cell mediated immunity.
Prototheca spp. grow rapidly on PDA after incubation at 30°C for 2-3 days. Initially, it may be confused with a yeast based on plate morphology as they are cream colored and have a yeast-like consistency. When a lactophenol cotton blue prep is viewed under the microscope, sporangia of various sizes are identified (3-30 µm) that contain endospores arranged in a “soccerball” or symmetric daisy like pattern. Size of the sporangia, assimilation tests (such as the API 20C) and automated yeast identification systems (such as the Vitek yeast ID by bioMérieux) are helpful in identifying Prototheca to a species level. Both P. wickerhammii (4-11 µm in diameter) and P. zopfii (9-28 µm in diameter) can cause disease in humans, but P. wickerhammii is more common. While P. wickerhammii has symmetric internal divisions as seen in the above image, P. zopfii has random internal divisions.
The treatment of P. wickerhammii usually includes a combination of surgical management and anti-fungals depending on the site of involvement. For superficial skin infections, localized excision and the use of topical or systemic anti-fungals (azoles and amphotericin B) has been shown to have success. The treatment for olecranon bursitis focuses on bursectomy. In the case of systemic infections, amphotericin B has been the most successful treatment modality. Susceptibility testing is not routinely recommended to guide treatment of initial infections, as studies are few and results don’t always correlate with outcome.
Lisa Stempak, MD, is an Assistant Professor of Pathology at the University of Mississippi Medical Center in Jackson, MS. She is certified by the American Board of Pathology in Anatomic and Clinical Pathology as well as Medical Microbiology. Currently, she oversees testing performed in both the Chemistry and Microbiology Laboratories. Her interests include infectious disease histology, process and quality improvement and resident education.